The present invention deals with improvements to the hydrogen embrittlement resistance of high strength nickel base columnar grain and equiaxed materials. The same principles which provide the improvements to hydrogen embrittlement resistance would also be expected to provide significant benefits to the fatigue behavior of the materials when used in an air atmosphere.
High strenth nickel base superalloys are defined in the context of this invention as nickel base alloys containing more than about fifty volume per cent of the strengthening .gamma.' phase in a .gamma. matrix and having yield strength in excess of 100 ksi at 1000.degree. F. Such alloys find their widest, and heretofore almost exclusive, application in the field of gas turbine engines. To the best of our knowledge, hydrogen embrittlement has only infrequently been a limiting factor in the performance of high strength nickel base superalloys.
In gas turbine engines, hydrocarbon fuels are burned, and free hydrogen may be present at some points during the combustion process, but the relatively low concentration of available hydrogen, and the operating conditions of such engines, have not been found to cause any significant hydrogen embrittlement of the nickel base superalloys.
Recently, however, in the development of the space shuttle main engines, hydrogen embrittlement has been recognized to be a significant problem. The space shuttle main engines are rocket engines which mix and react liquid hydrogen and liquid oxygen to form the propellant. These reactants are pumped into the main combustion chamber by turbo pumps which are powered by the combustion products of the reaction of hydrogen and oxygen. The hot side of the turbo pumps, which is exposed to the combustion products of the hydrogen/oxygen reaction, includes a multiplicity of small turbine blades which are investment cast from directionally solidified Mar-M 246 +Hf alloy, an alloy which meets the previous definition of a high strength nickel base superalloy in that it contains more than fifty volume per cent of the .gamma.' phase and has a yield strength of more than 100 ksi at 1000.degree. F. The nominal composition of Mar-M 246 +Hf is 9 Cf, 10 Co, 2.5 Mo, 10 W, 1.5 Ta, 5.5 Al, 1.5 Ti, 1.5 Hf, balance Ni, where each standard chemical symbol represents the weight percentage of the corresponding element. Hydrogen embrittlement of these turbine blades is a problem of great concern and is one of the factors which requires the space shuttle main engine pumps to be rebuilt with substantially greater frequency than originally anticipated.
Hydrogen embrittlement has been most commonly encountered in other fields of metallurgy, involving other metals and other environments. Hydrogen embrittlement occurs at times during electroplating, where hydrogen gas is generated directly on the surface of the part being plated and is absorbed into the part, greatly reducing the ductility of the part. Hydrogen embrittlement is also a factor in some forms of hot corrosion, especially hot corrosion which is observed in oil well drilling wherein deep drilled oil well casings are prone to hydrogen embrittlement as a result of the hydrogen sulfide present in some of the crude petroleum and natural gas which pass through the casings. U.S. Pat. Nos. 4,099,992, 4,421,571 and 4,245,698 are typical of the attempts to solve oil well hydrogen embrittlement problems.
Hydrogen embrittlement is encountered in these and other circumstances, and, while the exact mechanism involved is still open to conjecture, the existence of the problem is well documented. Initiation of hydrogen embrittlement cracking in nickel base superalloys has been found to occur at discontinuities in the structure, such as pores, hard particles and interfaces between precipitated phases and the matrix, such as script type carbides and .gamma./.gamma.' eutectic islands. Fatigue crack initiation has also been observed at similar sites in equiaxed superalloy materials, such as PWA 1489, which has a nominal composition of 8.4 Cr, 10 Co, 0.65 Mo, 5.5 Al, 3.1 Ta, 10 W, 1.4 Hf, 1.1 Ti, 0.015 B, 0.05 Zr, balance Ni, with all quantities expressed in weight percent. Strong evidence has been observed for the occurrence of interphase cleavage at the interfaces between the .gamma. matrix and .gamma.' particles, and within .gamma./.gamma.' eutectic islands. These features have been identified as fatigue crack initiation sites in this class of alloys in hydrogen.